Conjugated polymers are known to be utilized in varying electronic devices such as organic transistors, organic light emitting diodes (OLEDs) and organic photovotaic (PV) devices.
Polymer light emitting diodes (POLEDs) have been the subject of much interest and development over recent years, particularly in the field of display technology. The interest has been sparked owing to the many intrinsic advantages possessed by POLEDs over well-known liquid crystal display (LCD) devices. The significant advantage is the ability of the polymers to emit light, such that a separate light source is not required. This contrasts with the existing LCD devices in which an external light source has to be filtered in several stages in order to produce the final image. Thus, POLEDs also do not require any additional elements such as backlights and filters because they comprise polymer material which is manufactured on a transparent substrate of glass or plastic. POLEDs also have the benefit of being highly energy efficient, which makes them suitable candidates for low voltage operated ultra-thin lighting displays. POLEDs have the further benefit of being solution processable, enabling their formation by printing or solution casting techniques. One widely used class of polymers those comprising fluorene repeat units as disclosed in, for example, EP 0842208.
Presently known POLEDs can suffer from a limited lifetime, in particular as the polymers can be susceptible to a number of degradation pathways, comprising chemical, photochemical and electrochemical degradation. This can affect both the efficiency and the lifetime of the OLED. (By “lifetime” as used herein is meant the time taken for luminance to fall by 50% at constant current.) In the case of polyfluorenes, degradation may be the result of reactions at 4- and 5-positions of the fluorene unit.
Park et al, Adv. Funct. Mater, 2007, 17, 3063-3068 discloses poly(2,6-(4,4-bis(2-ethylhexyl)-8,9-dihydro-4H-cyclopenta-[def]-phenanthrene)) for the purpose of comparison with its dehydrogenated analogue poly(2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta-[def]phenanthrene)), and comparison with polyfluorene. The authors suggest that this material is susceptible to easier degradation than the two materials it is compared with.